1. Field of the Invention
The present invention relates to a screw rotor machine for compressing or expanding a compressible fluid and supplying the compressed or expanded fluid, and is particularly characterized by a toothprofile curve thereof.
The rotary machines termed in the present invention generally include a compressor, an expanding machine, a fluid motor, an internal combustion engine and the like.
2. Discussion of the Background
Rotors having nonsymmetrical tooth profiles and used in a compressor or the like of a compressible fluid generally comprise a male rotor having helical lands with a major portion of each tooth profile outside the pitch circle thereof, and a female rotor having helical grooves with a major portion of each tooth profile inside the pitch circle thereof. Normally, the male rotor has a plurality of teeth, and the female rotor meshing therewith has a number of teeth slightly exceeding the number of teeth of the male rotor. The diameter of the tip circle of the male rotor is set to be substantially the same as that of the pitch circle of the female rotor.
A screw compressor or expander is constructed as follows. A pair of screw rotors of this type are rotatably housed inside a cylinder space comprising two cylindrical bores formed in a casing. The cylindrical bores have parallel axes and have diameters equal to the outer diameter of the respective rotors to be arranged therein. The distance between the axes of the cylinders is shorter than the sum of the radii thereof, and the axial length of each cylindrical bore is the same as that of the rotors. The two end portions of the cylindrical bores are closed with end plates fixed to the casing. Inlet and outlet ports for the fluid are formed at predetermined positions of the casing.
When the above assembly is used as a compressor, the female rotor and the male rotor are respectively rotated in the opposite direction with respect to each other. With respect to the concave tooth profile of the groove of the female rotor, a curve at the front side along the rotating direction is referred to as the leading side tooth profile, and that at the rear side along the rotating direction is referred to as the trailing side tooth profile. Similarly, with respect to the convex tooth profile of the land of the male rotor, that at the front side along the rotating direction is referred to as the leading side tooth profile, and that at the rear side along the rotating direction is referred to as the trailing side tooth profile.
When the above assembly is used as an expander, the names of the respective curves are reversed. However, in the description of the present invention, the respective tooth profile curves will be explained in accordance with the above definitions.
FIGS. 3(a) and 3(b) show the respective tooth profile curves when the rotors are cut along a plane perpendicular to their rotating axes. FIG. 3(a) shows the phases of the tooth profiles of the two rotors immediately after the trailing side tooth profile curves of the male and female rotors have begun to contact each other. When the male rotor is rotated through about 20.degree. thereafter, the phases as shown in FIG. 3(b) are obtained wherein the highest portion of the tooth profile of the male rotor opposes the deepest portion of the groove of the tooth profile of the female rotor.
The above tooth profiles are ones disclosed in U.S. Pat. No. 4,576,558 granted to the present inventors and have the following characteristics.
Referring to FIGS. 3(a) and 3(b), reference numeral 1 denotes a male rotor; and 2 a female rotor meshed therewith. The rotors 1 and 2 rotate about centers of rotation 3 and 4 (centers of the pitch circles) inside cylindrical bores of a casing (not shown) in the direction indicated by arrows, respectively, so as to serve as a fluid compressor. Reference numerals 15 and 16 respectively denote pitch circles of male rotor 1 and female rotor 2. A line connecting the centers of rotation 3 and 4 passes a contact point 17 between the pitch circles 15 and 16, i.e., a pitch point 17.
The above-mentioned tooth profiles disclosed in U.S. Pat. No. 4,576,558 will be described with reference to FIG. 3(b).